Season the pike-perch fillets with salt and pepper and fry them in a frying pan with olive oil and a tiny bit of butter on both sides for about 3 minutes each.
Clean the leek and cut into fine rings. Fry the leek rings in a frying pan with clarified butter for 5 min. and season with salt and pepper.
Add the tarragon, fish stock, egg yolks, Noilly Prat, juice of one lemon and lemon zest to a round-bellied saucepan. Place the saucepan on the hot hob and whisk the sauce until it thickens, stirring continuously. Now pull the saucepan off the stove on the spot and whisk in the olive oil until the bottom of the saucepan has cooled. Season with salt and pepper, transfer the sauce to a sauce boat and serve on the spot.
Arrange the pike perch with the leek vegetables and offer the sauce with it.
By the way: Did you know that clarified butter is obtained by extracting water, milk protein and lactose from butter?